If you would write what cards you have, i would be glad to help you with building a Nightmare deck.

first piece of advice: Sargh and Erzsebeth are two cheap cards that will help you win rounds without outpilling your opponent, something you need in a Nightmare deck, as your power is generally not that great.

Mojo is a good supplement, to begin with, because your lack of dammage forces you to considder lifegain abilities, later however you would do well to find a card with higher power to replace her.